James Hansen, former NASA climate scientist, and three other prominent climate scientists are calling for an enlarged focus on nuclear energy in the ongoing Paris climate negotiations.

"Nuclear, especially next-generation nuclear, has tremendous potential to be part of the solution to climate change," Hansen said during a panel discussion yesterday. "The dangers of fossil fuels are staring us in the face. So for us to say we won't use all the tools [such as nuclear energy] to solve the problem is crazy."

He was joined by Tom Wigley, a climate scientist at the University of Adelaide; Ken Caldeira, a climate scientist at the Carnegie Institution for Science; and Kerry Emanuel, a professor of atmospheric science at the Massachusetts Institute of Technology.

Their stance clashes with those of environmental groups such as Greenpeace that advocate against nuclear energy.

As nations have proposed emissions curbs in Paris up to 2030, scientists have computed that there is a 1-in-2 chance that their collective ambition would raise temperatures in 2100 by between 2.7 to 3.7 degrees Celsius. Nations would like to limit temperature rise to 2 degrees Celsius, and stabilize atmospheric carbon dioxide levels at 450 parts per million (ppm).

There is 400 ppm of CO2 in the atmosphere at present.

So scientists have now turned their attention to what would be needed after 2030 to meet a 2 C goal: an energy system transformation that emits less carbon. For this, all technology options need to be on the table, including nuclear, the scientists said.

At present, there is a worrisome groundswell of opinion that renewable energy is sufficient to hit that target, Wigley of the University of Adelaide said. He is the owner of a zero-asset company, South Australian Nuclear Energy Systems, that educates people on the technology but is not involved with the nuclear industry.

"We are alarmed by people who want to close the door on nuclear, and so that is why we are more outspoken than we might have been a few years ago," he said in a phone interview.

Very few nations, at present, mention nuclear in their greenhouse gas emissions reduction pledges, he said. Given the long time needed to build a nuclear power plant, nations should prioritize the technology immediately, he said.

The scientists stressed that even a 2 C target might not be effective. Hansen has previously emphasized that sea-level rise could threaten coastal areas even if that target is met.

Can new nukes be a cheaper alternative?
If nations meet their Paris pledges, called intended nationally determined contributions, or INDCs, and continue decarbonizing beyond 2030 at a rate of 5 percent, there is more than a 3-in-4 chance that a 4 C temperature rise could be avoided. That much warming could trigger irreversible tipping points in the Earth system and catastrophic climate change. The findings were published in Science.

The United States' INDC up to 2030 would require the nation to decarbonize at a rate of 6 percent. China will have to decarbonize at 4 percent.

The Paris pledges make it more probable than before that nations will meet 2 C, provided that the world decarbonizes rapidly after 2030, said Allen Fawcett, chief of U.S. EPA's Climate Economics Branch and lead author of the study. Nations are negotiating in Paris mechanisms to review their climate goals every five years and ratchet up their ambitions.

"Paris is a steppingstone to a better climate future," Fawcett said. "Each additional contribution and each additional increase in ambition that countries make under the Paris framework will help improve our chances of limiting future warming."

Beyond 2030, nations would need a portfolio of technology options to decarbonize, said Gokul Iyer, a researcher with the Pacific Northwest National Laboratory's (PNNL) Joint Global Change Research Institute and a co-author of the study.

"That is going to entail premature retirements of fossil fuel power plants, and also additional renewable, nuclear and carbon capture and sequestration power plants," Iyer said.

If the nuclear energy option is ignored, nations would have to pay a larger bill to achieve their goals, Fawcett said.

"The more technology is available and the more different opportunities you have for reducing emissions, the less costly those pathways [to 2 C] tend to be," he said.

Or can renewables go it alone?
Mark Jacobson, a professor of civil and environmental engineering at Stanford University, is optimistic that the world can meet the 2 C target and, in fact, stabilize emissions at 350 ppm instead of the 450 ppm that the United Nations aspires to, using solely renewable energy.

The technologies for this transformation -- wind, water and solar energy -- already exist, he said. They could entirely replace the world's fossil fuel-based energy system by 2050, if governments will it to be so, he said.

"The only obstacles are social and political," he said. "The only reason why it can't get implemented is because there are people against it."

In Jacobson's energy matrix, nuclear energy does not play a role. Nuclear plants need two decades to build, and the mining of uranium fuel is carbon-polluting, he said.

"It is a a whole distraction, and people should know better than to propose nuclear energy, because people who are working in this field know it is not going to go anywhere," he said.

Instead, Jacobson proposes that the world overcome its sociopolitical barriers and install 80 percent renewables by 2030 and 100 percent by 2050. During times when the wind does not blow or the sun does not shine, he proposes using hydropower to make up the gap.

He said the costs of the transformation would be worth the benefits: 22 million net jobs, the costs of global warming, avoiding unstable energy prices and energy security.

"We can have 100 percent reliable grid across the U.S. without nuclear, without natural gas, without biofuels, with only wind, water, solar, with low-cost storage," he said.

Quick, factory-built nuclear power plants?
Other scientists would like to see more research and development to bring down the costs of the energy transition. The Bill Gates-led Breakthrough Energy Coalition last week announced a $2 billion fund for clean energy research.

"Technological change is going to be a critical element in controlling costs of achieving these stringent targets," Iyer of PNNL said.

Wigley of the University of Adelaide sees improvements on the horizon for nuclear technologies, particularly in China, where modifications of Westinghouse technology could allow new plants to be set up in two to three years.

"There are technological innovations in the wings at the moment that will make it much quicker to build nuclear power stations," he said. "There are technologies that involve modular systems where the components for a large number of power stations can be built in a factory and taken to a site and assembled together."

Every attempt to increase national ambitions would help the world meet the 2 C target, Fawcett of EPA said.

"[It] will have a real and tangible benefits in terms of improving the odds of a better climate outcome, reducing the chance of extreme outcomes, improving our changes of limiting the warming to the lowest levels we can," he said.

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